In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and lithographic printing ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is eventually transferred to the surface of a material upon which the image is to be reproduced.
Lithographic printing precursors useful for preparing lithographic printing plates or sleeves typically comprise one or more imagable layers applied over the hydrophilic surface of a substrate. The imagable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imagable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is considered as positive-working. Conversely, if the non-imaged regions are removed, the precursor is considered as negative-working. In each instance, the regions of the imagable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Lithographic printing precursors for the preparation of lithographic printing plates have been developed for use with infrared lasers that image in a platesetter in response to signals from a digital copy of the image in a computer. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the precursors.
Early lithographic printing plate precursors for use with infrared lasers typically involve an ablation process, which selectively remove ink receptive or ink repellant surface materials and thereby revealing surfaces of complementary ink affinity. Some examples of such ablative lithographic printing plate precursors are described in U.S. Pat. No. 5,339,737 (Lewis et al.). These lithographic plate precursors typically require a relatively high amount of energy input and thereby have limited productivity when the available infrared laser power is limited. Another drawback of ablative lithographic printing plate precursors is the need to remove debris during imaging, adding to the cost of the imaging equipment and more noise during the operation of such equipment. The first commercial non-ablative lithographic printing plate precursors for use with infrared lasers required a preheat step between infrared laser imaging and image development. Some examples of such lithographic printing plate precursors requiring a preheat step are described in U.S. Pat. No. 5,372,907 (Corbiere et al.). Due to the extra equipment and energy consumption required by the preheat step, lithographic printing plate precursors that do not require a preheat step were designed as described for example in U.S. Pat. No. 6,280,899 (Parsons et al.) and U.S. Pat. No. 6,326,122 (Nagasaka et al.). Such lithographic printing plate precursors are typically positive-working and contain an infrared laser imagable layer comprising novolac resins as the primary binders. These lithographic printing plate precursors typically have limited durability on press and limited resistance to press chemicals, unless such plates are baked at high temperature after image development. The baking step adds to equipment cost and energy consumption.
Various attempts have been made to improve the run length on press and chemical resistance of the no-preheat plates, but each attempt has one or more limitations. For example, no-preheat lithographic printing plate precursors containing some acrylic binders, described in U.S. Pat. No. 6,143,464 (Kawauchi) typically suffer from inadequate differentiation in the solubility in alkaline developers between the IR laser exposed areas and non-exposed areas and from such problems like poor scratch resistance. This inadequate image differentiation often leads to excessive coating loss in the non-IR exposed areas or incomplete removal of the coatings from the IR exposed areas. The image development conditions such as developer strength, temperature, developing time, and brush pressure need to be tightly controlled. Therefore, such lithographic printing plate precursors are considered to have narrow development latitude.
Two-layer no-preheat lithographic plate precursor are described in U.S. Pat. No. 6,294,311 (Shimazu et al.). These lithographic printing plate precursors require manufacturing lines capable of providing two coated layers. Therefore, there is an ongoing desire to develop single-layer, no-preheat, positive-working lithographic plate precursors that exhibit good run length on press and good resistance to press chemicals.
In search of such precursors, it was found that single-layer positive-working lithographic printing plate precursors containing a poly(vinyl acetal) have excellent run length on press and excellent development latitudes. Some examples of such printing plate precursors are described in U.S. Pat. No. 7,399,576 (Levanon et al.) and U.S. Pat. No. 7,544,462 (Levanon et al.) and U.S. Patent Application Publications 2006/0154187 (Wilson et al.) and 2009/0162783 (Levanon et al.). However, there is a continuing need to improve their resistance to certain press chemicals and solvents.
It was found that solvent resistance could be improved using a poly(vinyl acetal) that also includes recurring units having hydroxyaryl ester groups, as described for example, in copending and commonly assigned U.S. Ser. No. 12/555,040 (filed Sep. 9, 2009 by Levanon, Bylina, Kampel, Rubin, Postel, Kurtser, and Nakash). While good run length and solvent resistance were obtained with these plates, there is a continuing need to improve the development latitude with developer compositions.
A specific developer composition is often optimized for developing a particular positive-working lithographic printing plate precursor. There have been attempts to do this by including coating protecting agents in the developer to reduce the solubility of the imagable coating in the non-exposed areas more effectively than the imagable coating in the exposed areas.
One cause of short development cycle and excessive difficulty in cleaning the automatic processor relates to partial dissolution of aluminum oxide film on the substrates of typical lithographic printing plate precursors in the developer solution. Techniques for reducing or eliminating such aluminum oxide attacks include the use of alkali silicates, non-reducing sugars, or lithium salts such as lithium chloride. However, the use of silicate salts itself adds to the dirtiness of the processor bath. It was found that developers containing lithium chloride are very slow in dissolving the infrared laser exposed coating containing polyvinyl acetal that also has hydroxyaryl ester groups and therefore are considered unsuitable for processing such precursors.
These problems are addressed using the method described and claimed in copending and commonly assigned U.S. Ser. No. 12/948,808 filed on Nov. 18, 2010 by Levanon, Huang, and Askadsky. Improved image discrimination was achieved with the described lithographic printing plate precursors by processing them using a developer composition having a pH of at least 12 and comprising at least 0.001 gram-atom/kg of a metal cation M2+ such as barium, calcium ions, strontium, and zinc cations.
Other useful developer compositions are described and claimed in copending and commonly assigned U.S. Ser. No. 12/948,814 filed on Nov. 18, 2010 by Levanon and Askadsky.
The presence of M2+ cations such as calcium ions in the developer composition also acts to protect the aluminum substrate from attack by the highly alkaline developer.
During a processing cycle, when the developer is “loaded” with dissolved coating materials, a problem known as “sharpening” become evident. “Sharpening” occurs when the developing composition becomes more aggressive in its developing activity so that the non-exposed regions in the imagable layer are attacked by the developer composition, resulting in increased printing plate weight loss and decreased dot size (“dot sharpening”) in the resulting printed imaged.
Thus, while the inventions described in the noted copending and co-filed applications solves certain problems, there is an additional need to find a way to maintain stable performance for a long time during high loading of the developer composition with dissolved coating materials from the processed lithographic printing plate precursors.